Method of producing a lead frame with composite film attached, and use of the lead frame

ABSTRACT

Disclosed is a composite film, a method of producing a lead frame with the composite film attached, and a method of bonding a semiconductor chip to this lead frame having the composite film attached. The composite film includes a base film and an adhesive layer on one or both sides of the base film. The composite film has a thickness of T (μm), the base film has an edge tearing strength of R (kg/20 mm), the adhesive layer has a total thickness of A, and the base film has a thickness of B, T being related to R by a numerical formula R&gt;0.6T-8 when T≦60, or by a numerical formula R≧28 when T&gt;60, and A/B being 0.5 to 1.4.

[0001] This application is a Divisional application of prior applicationSer. No. 09/421,002, filed Oct. 20, 1999, which is a Divisionalapplication of prior application Ser. No. 08/844,430, filed Apr. 18,1997.

BACKGROUND OF THE INVENTION

[0002] (a) Field of the Invention

[0003] The present invention relates to a composite film which hasexcellent punching capability and is useful as a bonding element forsemiconductor packaging.

[0004] (b) Description of Related Art

[0005] In semiconductor packaging, a bonding element which is acomposite film produced by coating one or both sides of a base film withadhesives is used for bonding chips to lead frames to form LOC (lead onchip) or COL (chip on lead) structures or window-tub structures, or forbonding inner leads to heat spreaders to form composite lead frames withheat sinks attached. The bonding element is generally applied to thelead frames by punching out the bonding element from the composite filmover the lead frames and pressing the bonding elements to required partsof the lead frames. However, the yield rate of the resulting lead frameswith composite film attached is low since flashes are frequently formedat the punched edges of the composite film.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide a composite filmwhich is so excellent in punching capability as not to form flashes atthe time of punching and are useful as bonding film in semiconductorpackaging.

[0007] Another object of the present invention is to provide a leadframe with composite film bonded to the required parts of the lead frameby punching the composite film of the present invention and is reliablewhen used in semiconductor packages.

[0008] As a result of studies in the relationship between the propertiesof composite film and flashes, we have found that flashing can beeffectively prevented when there exists specific relation between thethickness T of composite film and the edge tearing strength R and havecompleted the present invention based on the finding.

[0009] That is, the present invention provides a composite film,comprising a base film and an adhesive layer on at least one side of thebase film, the composite film having a thickness of T (μm), the basefilm having an edge tearing strength of R (kg/20 mm), the adhesive layerhaving a total thickness of A, the base film having a thickness of B, Tbeing related to R by a numerical formula R>0.6T-8 when T≦60, or by anumerical formula R≧28 when T>60, and A/B being 0.5 to 1.4.

[0010] The present invention further provides a lead frame with acomposite film attached, comprising a lead frame body and a compositefilm applied to the lead frame body, the composite film being punchedout from the composite film of the present invention.

[0011] The present invention further provides a method of producing alead frame with a composite film attached, comprising punching out acomposite film from the composite film of the present invention,pressing the punched out composite film to a lead frame body, with oneadhesive layer contacting a surface of the lead frame body.

[0012] Herein, the edge tearing strength of base film is measuredaccording to JIS C 2318.

[0013] Flashes, which tend to be formed at the edges of punchedcomposite film comprising a base film and an adhesive layer on at leastone side of the base film, can be extremely decreased by using acomposite film wherein T and R are related to each other by the abovenumerical formula and the ratio of the thickness A of the adhesive layerto the thickness B of the base film, A/B, is within the above-describedrange.

THE PREFERRED EMBODIMENTS OF THE INVENTION

[0014] The composite film of the present invention is produced byapplying adhesives to one or both sides of a base film, followed bydrying. The adhesives applied to both sides of the base film may beidentical with or different from each other.

[0015] The desirable adhesives are heat resistant adhesives whichcontain as main components heat resistant thermoplastic resins and havea glass transition temperature (Tg) of 150 to 350° C., a waterabsorption of 3% or less and a broadening length of 2 mm or less, and,therefore, polyimide adhesives and polyamide adhesives are suitable.

[0016] Herein, the term “polyimide” means not only polyimide but alsoany other resin containing imide bonds, such as polyamideimide,polyesterimide or polyetherimide.

[0017] If the glass transition temperature is beyond the above-describedrange, or the water absorption is more than 3% by weight, or thebroadening length is longer than 2 mm, the resistance to reflow crackingof packages tends to be decreased.

[0018] The water absorption of the heat resistant adhesives is morepreferably 2.5% by weight or less, particularly preferably 2.0% byweight or less. The broadening length is more preferably 1 mm or less,particularly preferably 0.5 mm or less.

[0019] In addition, fillers, such as ceramic powder, glass powder,silver powder, copper powder, resin powder, gum powder and couplingagents may also be added to the heat resistant adhesives. The heatresistant adhesives may also be used after impregnating with it a basesheet, such as glass fabric, aramid fabric or carbon fiber fabric.

[0020] Usable examples of the coupling agents include; vinylsilanes,such as vinyltriethoxysilane, vinyltrimethoxysilane andγ-methacryloxy-propyltrimethoxysilane; epoxysilanes, such asγ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilaneand β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; aminosilanes, such asγ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane andN-phenyl-γ-aminopropyltrimethoxysilane; mercaptosilanes, such asγ-mercaptopropyltrimethoxysilane; and coupling agents such as titanates,aluminum chelates and zircoaluminates.

[0021] Of these, silane coupling agents are preferred with epoxysilanecoupling agents being particularly preferred.

[0022] Herein, “broadening length of an adhesive” defines a width bywhich a film of the adhesive of 19×50 mm and 25 μm thick is broadenedout perpendicularly from the middle of each side of the film by pressingthe film at 350° C. at 3 MPa for one minute.

[0023] The base film to be used in the present invention, preferably, isa heat resistant film made of an engineering plastic, such as polyimide,polyamide, polysulfone, polyphenylenesulfide, polyetheretherketone orpolyarylate. The thickness of the base film is preferably 5 to 150 μm,more preferably 20 to 125 μm.

[0024] The glass transition temperature (Tg) of the heat resistant filmis preferably higher than the Tg of the adhesives to be used in thepresent invention, and is preferably 200° C. or higher, more preferably250° C. or higher. The heat resistant film preferably has a waterabsorption of 3% by weight or less, more preferably 2% by weight orless.

[0025] Therefore, polyimide film is suitable as the base film to be usedin the present invention because of the high Tg, the low waterabsorption and the low thermal expansion coefficient thereof. Theparticularly preferred film has a Tg of 250° C. or higher, a waterabsorption of 2% by weight or less and a thermal expansion coefficientof 3×10⁻⁵/°C. or less.

[0026] To increase the adhesion force between the base film and theadhesives, it is preferable to surface treating the base film. Thesurface treatments applicable in the present invention are not limitedand include, for example, chemical treatment, such as alkali treatmentand silane coupling agent treatment, physical treatment, such as sandblasting, plasma treatment or corona treatment, and it is possible toselect one or more treatments that are the most suitable for theadhesives used. In cases where heat resistant film is used as the basefilm and adhesive layers of heat resistant adhesive are applied thereto,chemical treatment or plasma treatment is particularly suitable.

[0027] The method of forming the adhesive layers on the base film is notparticularly limited, and an example of suitable methods comprisesapplying adhesive varnish to the base film and drying to removesolvents. The method of applying the adhesive varnish to the base filmis not particularly limited, and may be any one using a doctor blade, aknife coater or a dye coater. The application may also be performed bydipping the base film in adhesive varnish, but it may be difficult tocontrol the thickness of the adhesive layers.

[0028] For example, the adhesive varnish may be a varnish of a heatresistant resin, such as a heat resistant thermoplastic resin, or a heatresistant adhesive comprising mainly of a heat resistant resin,dissolved in solvents. Alternatively the adhesive varnish may be avarnish of a heat resistant resin precursor which can be converted intoa heat resistant resin by heat treatment or the like following tocoating, or an adhesive composition comprising mainly of the precursor,dissolved in solvents. Examples of the precursors are polyamic acids,which can be converted into polyimides by heat treatment.

[0029] When adhesive varnish applied to the base film is subjected toheat treatment for the removal of solvents or for conversion into imide,polyamic acid varnish and polyimide varnish are treated at differenttemperatures. Polyamic acid varnish is preferably treated at atemperature not lower than the Tg of polyimides to perform theconversion into the polyimides, while polyimide varnish may be treatedat any temperature sufficient to remove solvents.

[0030] After the adhesive layers are formed on the base film, it ispreferable to conduct heat treatment at a temperature of 250° C. orhigher for 1 to 30 minutes to improve the adhesion force between theadhesive layers and the base film.

[0031] The thickness of each adhesive layer formed on the base ,film ispreferably 5 to 50 μm, more preferably 10 to 30 μm.

[0032] The lead frame of the present invention comprises a lead framebody and composite film applied to the lead frame body, the compositefilm being punched out from the composite film of the present invention.

[0033] The lead frame bodies may have any structure and, for example,comprise inner lead portions to be connected to a semiconductor chip,outer lead portions to be connected to external circuits, and thecomposite film of the present invention is applied to predeterminedportions of the lead frame bodies.

[0034] The lead frames with composite film attached may be produced bybonding the composite film to lead frame bodies by punching thecomposite film onto the lead frame bodies. Semiconductor packagesproduced by using the lead frames with composite film attached areexcellent in reliability.

[0035] For example, the bonding of the composite film to lead frames maybe efficiently performed by placing the composite film over a lead framebody, with one adhesive layer facing the lead frame, punching thecomposite film into strips by using punching metal molds, whichcontinuously press the punched out strips to predetermined portions ofthe underlying lead frame body at a pressure of 0.1 to 10 MPa for 0.1 to5 seconds, to bond them by their adhesive layers. The forms of thepunched out strips of the composite film vary, for example, depending onthe forms of chips, the locations of pads on chips or the designs oflead frames. At the time of punching, the lead frame body is generallyheated to a predetermined temperature, for example, 200 to 500° C. Thecomposite film to be punched may also be heated. If flashes are formedaround the punched strips, the flashes adhere to bonding areas of leadframes, to interfere wire bonding.

[0036] Hereinafter, the present invention will be described in detailreferring to examples, but the scope of the invention is not limited bythe examples.

EXAMPLE 1

[0037] Each side of a polyimide film (Trade name: UPILEX-S, produced byUbe Industries, Ltd., Tg>300° C., water absorption: 1.2% by weight,thermal expansion coefficient: 1.6×10⁻⁵/°C.) having an edge tearingstrength of 35 kg/20 mm and a thickness of 25 μm was coated with a 17 μmthick layer of a polyamideimide adhesive (Trade name: HIMAL, produced byHitachi Chemical Co., Ltd.) having a Tg of 185° C., a water absorptionof 1.7% by weight and a broadening length of 0.05 mm, to produce acomposite film of a total thickness of 59 μm.

[0038] The composite film was placed over a lead frame of 42 ally heatedto 400° C., and was then punched by using punching metal molds to formpunched out strips, which were then continuously pressed by the punchingmetal molds to leads of the underlying lead frame for one second at apressure of 3 MPa, to give a lead frame with composite film attached. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

COMPARATIVE EXAMPLE 1

[0039] A composite film was produced in the same manner as in Example 1with the exception that a polyimide film (UPILEX-S) having an edgetearing strength of 25 kg/20 mm and a thickness of 25 μm was used.

[0040] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1.Flashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 2

[0041] Each side of a polyimide film (UPILEX-S) having an edge tearingstrength of 30 kg/20 mm and a thickness of 50 μm was coated with a 25 μmthick layer of the same polyamideimide adhesive as that used in Example1, to produce a composite film of a total thickness of 100 μm.

[0042] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 3

[0043] Each side of a polyimide film (UPILEX-S) having an edge tearingstrength of 30 kg/20 mm and a thickness of 25 μm was coated with a 17 μmthick layer of a polyamideimide adhesive having a Tg of 225° C., a waterabsorption of 1.8% by weight and a broadening length of 2.0 mm, toproduce a composite film of a total thickness of 59 μm.

[0044] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 4

[0045] Each side of a polyimide film (UPILEX-S) having an edge tearingstrength of 35 kg/20 mm and a thickness of 50 μm was coated with a 25 μmthick layer of the same polyamideimide adhesive as that used in Example3, to produce a composite film of a total thickness of 100 μm.

[0046] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 5

[0047] Each side of a polyimide film (UPILEX-S) having an edge tearingstrength of 30 kg/20 mm and a thickness of 50 μm was coated with a 15 μmthick layer of the same polyamideimide adhesive as that used in Example3, to produce a composite film of a total thickness of 80 μm.

[0048] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

COMPARATIVE EXAMPLE 2

[0049] Each side of a polyimide film (UPILEX-S) having an edge tearingstrength of 25 kg/20 mm and a thickness of 25 μm was coated with a 20 μmthick layer of the same polyamideimide adhesive as that used in Example1, to produce a composite film of a total thickness of 65 μm.

[0050] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1. Agreat many flashes were observed by microscopic examination of the edgesof the punched composite film.

COMPARATIVE EXAMPLE 3

[0051] A composite film having a total thickness of 65 μm was producedin the same manner as in Comparative Example 2 with the exception that apolyimide film (UPILEX-S) having an edge tearing strength of 30 kg/20 mmand a thickness of 25 μm was used.

[0052] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1. Manyflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 6

[0053] Each side of a polyimide film (UPILEX-S) having an edge tearingstrength of 25 kg/20 mm and a thickness of 25 μm was coated with a 12 μmthick layer of the same polyamideimide adhesive as that used in Example1, to produce a composite film of a total thickness of 49 μm.

[0054] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 7

[0055] Each side of a polyimide film (UPILEX-S) having an edge tearingstrength of 30 kg/20 mm and a thickness of 25 μm was coated with a 15 μmthick layer of the same polyamideimide adhesive as that used in Example1, to produce a composite film of a total thickness of 55 μm.

[0056] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

COMPARATIVE EXAMPLE 4

[0057] A composite film having a total thickness of 55 μm was producedin the same manner as in Example 7 with the exception that a polyimidefilm (UPILEX-S) having an edge tearing strength of 20 kg/20 mm and athickness of 25 μm was used.

[0058] A lead frame with composite film attached was produced in thesame manner as in Example 1 with the exception that the composite filmmade as above was used in place of the composite film of Example 1.Flashes were observed by microscopic examination of the edges of thepunched composite film.

[0059] The values of T, R,(0.6T-8) and A/B in the above-describedExamples and Comparative Examples are shown in Table 1. TABLE 1 T R A BA/B (μm) (kg/20 mm) 0.6T-8 (μm) (μm) (0.5-1.4) Example 1 59 35 27.4 3425 1.36 Comp. 59 25 27.4 34 25 1.36 Example 1 Example 2 100  30 28 50 501.00 Example 3 59 30 27.4 34 25 1.36 Example 4 100  35 28 50 50 1.00Example 5 80 30 28 30 50 0.60 Comp. 65 25 28 40 25 1.60 Example 2 Comp.65 30 28 40 25 1.60 Example 3 Example 6 49 25 21.4 24 25 0.96 Example 755 30 25 30 25 1.36 Comp. 55 20 25 30 25 1.36 Example 4

[0060] As evident from the results as shown in Table 1, the compositefilm of the present invention has such an excellent punching capabilitythat no flashes are formed at the punched edges, and by using leadframes with the composite film attached, extremely reliablesemiconductor packages can be produced.

What is claimed is:
 1. A method of producing a lead frame with acomposite film attached, comprising steps of: placing a composite filmover a lead frame, the composite film including a base film and anadhesive layer on at least one side of the base film, such that thecomposite film has the at least one adhesive layer, the composite filmhaving a thickness of T (μm), the base film having an edge tearingstrength of R (kg/20 mm), the at least one adhesive layer having a totalthickness of A, and the base film having a thickness of B, T beingrelated to R by a numerical formula R>0.6T-8 when T≦60, or by anumerical formula R≦28 when T>60, and A/B being 0.5 to 1.4, with oneadhesive layer facing the lead frame, wherein the at least one adhesivelayer is made of an adhesive having a broadening length of 2 mm or less;punching the composite film into a strip by using a punching metal mold;and pressing the strip to the lead frame with the punching metal mold ata pressure of 0.1 to 10 MPa for 0.1 to 5 seconds.
 2. The method of claim1, wherein the lead frame is heated to 200° to 500° C. during saidpunching.
 3. The method of claim 1, wherein the adhesive layer is madeof an adhesive having a glass transition temperature of 150° to 350° C.and a water absorption of 3% or less.
 4. The method of claim 3, whereinthe base film is made of a material having a glass transitiontemperature higher than that of said adhesive, and having a waterabsorption of 3% or less.
 5. The method of claim 4, wherein the adhesiveis a polyimide adhesive or a polyamide adhesive.
 6. The method of claim4, wherein the base film is a heat resistant film made of a resinselected from the group consisting of polyimide, polyamide, polysulfone,polyphenylenesulfide, polyetheretherketone and polyarylate.
 7. Themethod of claim 4, wherein each side of the base film is coated with theadhesive layer, and the thickness A is the total thickness of theadhesive layers.
 8. The method of claim 4, wherein the base film has athickness of 5 to 150 μm, and each adhesive layer has a thickness of 5to 50 μm.
 9. The method of claim 4, wherein the glass transitiontemperature of the base film is at least 200° C.
 10. The method of claim1, wherein the broadening length is 0.5 mm or less.
 11. The method ofclaim 1, wherein the base film has a thickness of 20 to 125 μm, and eachadhesive layer has a thickness of 10 to 30 μm.
 12. The method of claim1, wherein R is 25 to
 35. 13. A method of bonding a semiconductor chipto a lead frame, comprising steps of: punching a composite film to punchout a strip of the composite film, the composite film including a basefilm and an adhesive layer on each side of the base film, the compositefilm having a thickness of T (μm), the base film having an edge tearingstrength of R (kg/20 mm), the adhesive layer having a total thickness ofA, and the base film having a thickness of B, T being related to R by anumerical formula R>0.6T-8 when T≦60, or by a numerical formula R≧28when T>60, and A/B being 0.5 to 1.4, and wherein the adhesive layer ismade of an adhesive having a broadening length of 2 mm or less; applyingthe strip to the lead frame by pressing the strip so that one adhesivelayer of the strip contacts the lead frame; and bonding a semiconductorchip onto another adhesive layer of the strip.
 14. The method of claim13, wherein the composite film is placed over the lead frame, with oneadhesive layer facing the lead frame body, and punched with a punchingmetal mold to punch out the strip of the composite film, and the stripis pressed to the lead frame with the punching metal mold at a pressureof 0.1 to 10 MPa for 0.1 to 5 seconds.
 15. The method of claim 13,wherein the lead frame is heated to 200° to 500° C. during saidpunching.
 16. The method of claim 13, wherein the adhesive layer is madeof an adhesive having a glass transition temperature of 150° to 350° C.and a water absorption of 3% or less.
 17. The method of claim 13,wherein the base film is made of a material having a glass transitiontemperature higher than that of said adhesive, and having a waterabsorption of 3% or less.
 18. The method of claim 13, wherein theadhesive is a polyimide adhesive or a polyamide adhesive.
 19. The methodof claim 13, wherein the base film is a heat resistant film made of aresin selected from the group consisting of polyimide, polyamide,polysulfone, polyphenylenesulfide, polyetheretherketone and polyarylate.20. The method of claim 13, wherein the base film has a thickness of 5to 150 μm, and each adhesive layer has a thickness of 5 to 50 μm. 21.The method of claim 13, wherein the glass transition temperature of thebase film is at least 200° C.
 22. The method of claim 13, wherein R is25 to 35.